Conventionally, as chains used in a ship windlass or a cargo-handling machine and so on, a substantially .theta.-shaped stud link 5 comprising O-shaped link 3 and stud 4 which connects the middle part of the link 3 is widely used for increasing the tensile shearing stress in the longitudinal direction or for preventing entanglement of the links. As shown in FIG. 5, this kind of chain is formed in such a manner that a round-bar type steel material 10 is heated up to 850.degree.-900.degree. C. by an electric resistance heater and then is bent into a C-shape and opening ends 31, 31 of the C shape are connected with each other by means of electric resistance welding, then flashing 32 in the welded part is removed and a stud 4 is inserted and fixed by pressing. Especially when strength in a stud part is required, usually CO2 gas arc welding is applied to the stud end fixed to the link.
In the above mentioned production methods, (1) a bar-type steel material having a uniform section is bent to an angle of 180.degree. C., and thereby the bent corner part (so called a shoulder part) comes to have a slightly smaller diameter compared with the other parts, resulting in that the tensile stress of the chain is susceptible to act on this shoulder part and fatigue failure starting from here easily takes place. Further, (2) according to the conventional method in which the studs are fixed by pressing, as a result of repeated use, deformation stress acts on the outer periphery of the link, or the studs may fall off due to the rust. According to the method in which an end of each stud is welded for fixation, it is disadvantageous that as a result of repeated use, the welded point is apt to cause fatigue failure due to the deformation stress acted on the outer periphery of the link. Especially for these chains used in conveyors or for the fishery purpose, in which the tensile strength is placed on the studs upon driving, even increased strength is required in the studs.
Moreover, (3) for the purpose of traction and support of marine structures, lightweight materials also showing a high tensile strength, such as SUS or chromium molybdenum for example, may be proposed. However, a problem of weldability arises and production according to the conventional methods is difficult.
Further, (4) each connected part in the chain wears due to the friction, as shown by the shadowed portion m in FIG. 6, namely, the service life of the chain is usually determined by the remaining diameter b. In the conventional chain, since the sectional shape is uniform, the diameter of the section must be increased to gain the service life of the chain. However this cannot be employed, since it accompanies with the significant increase in chain weight.
For the purpose of improving fatigue strength of this kind of chain, link chains according to Japanese Patent Tokkaisho No.55-31677 and No. 62-3848 have been proposed. In both link chains, however, a bar steel is formed into an O-link by bending the bar steel to a maximum angle of 180.degree. C., which remains the fundamental problem of fatigue failure due to shearing stress applied to the shoulder part upon pulling the chain in the longitudinal direction.